A single stage compressor with an overhung rotor for process gas having one end of the compressor shaft exposed to atmosphere and the other end exposed to suction pressure can incur an extremely large thrust load on the shaft as a result of the imposed pressure differential. At low operating speeds, such as at startup or shutdown, the imposed loading is known to exceed the load factor for which the thrust bearing was selected. For that reason it has been necessary to improvise in order to allow low speed operation when excessive thrust load is being incurred.
A conventional approach toward resolving the foregoing has been to increase the load capacity of the bearing sufficient to withstand the load levels which the bearing incurs. This obviously represents a costly and unsatisfactory solution. Another approach has been to reduce the pressure differential by venting the process gas from the compressor to reduce the overall internal pressure to an acceptable level. This approach has likewise been unsatisfactory in that it has resulted in considerable wastage of process gas. Still another technique has been to counteract the high pressure differential with appropriately directed high pressure gas or oil at a pressure approximating the process gas. The latter tends to complicate and enlarge the seal oil system or require outside sources of high pressure gas.
As an expedient, each of the foregoing approaches has fulfilled the objective of maintaining thrust loads within tolerable limits. On the other hand, none has been regarded as satisfactory despite long-standing recognition of the problem.